1. Field of the Invention
The present invention relates to cooling systems for cooling heat generating components in electronic equipment (electronics). It particularly relates to cooling systems which efficiently and stably cool heat generating components by utilizing the latent heat of vaporization of a liquid refrigerant while circulating the refrigerant.
2. Description of Related Art
Semiconductor devices such as CPUs (central processing units) in electronic equipment (electronics) typified by computers need to be temperature controlled (mostly by cooling) from the standpoint of performance stabilization and failure prevention of such electronic equipment. Conventionally, there have been generally employed a passive cooling system which regulates the air temperature surrounding electronic equipment using an air conditioner or the like, and cools the electronic equipment by utilizing heat dissipating fins attached to devices such as CPUs and ventilation fans. With recent trend toward higher packing densities and higher operation speeds, semiconductor devices generate more and more heat. There is also a demand for downsizing of electronic equipment, so active cooling systems for heat generating components in electronic equipment are receiving much attention today.
For example, JP-A-2005-228216 discloses a method of direct liquid cooling for heat generating components in electronic equipment, in which heat is directly transferred from the components to a liquid refrigerant used as heat exchange medium. JP-A Hei 10 (1998)-160368 and JP-A-2006-308163 disclose a method for downsizing a cooling system by utilizing a loop heat pipe.
However, the above-mentioned conventional methods have technical problems to be solved. In order to efficiently liquid cool semiconductor devices such as CPUs mounted on various blade modules of electronic equipment, the cooling system of the above-mentioned JP-A-2005-228216 is configured with: multiple cooling units for liquid cooling the respective blade modules; and a heat exchange unit for venting heated liquid refrigerants from the respective cooling units, cooling them with a heat exchanger and redistributing them to the multiple cooling units. This patent describes that efficient cooling can be achieved by regulating the flow rates of the liquid refrigerants distributed from the heat exchange unit to the cooling units. However, the cooling efficiency depends on the flow rate of liquid refrigerant and therefore a relatively large amount of liquid refrigerant is needed, thus possibly incurring an increase in the size of the liquid refrigerant feed pump and heat exchanger.
The heat pipe cooling system described in the above-mentioned JP-A Hei 10 (1998)-160368 uses a loop heat pipe in which the heat receiving and dissipating units and the piping connecting them are optimized in order to achieve downsizing and improved efficiency of the cooling system and to prevent dew formation (condensation) harmful to electronic components. This cooling system acquires a high cooling capacity by utilizing the latent heat of the working fluid of the heat pipe. However, because the circulating force is provided by difference in operating pressure between the heat receiving and dissipating units, the pumping force is by its nature small, so sufficient force for circulating the liquid refrigerant cannot be maintained in some situations (particularly at lower loads) Therefore, a pump is needed to overcome this problem. In addition, it is difficult to achieve a stable cooling capacity.
In order to improve the pumping force generated by the vaporization of the working liquid of the heat pipe, the cooling system of the above-mentioned JP-A-2006-308163 has, within the heat receiving container, multiple wicks each producing a capillary pressure by impregnating it with the working liquid. Each wick is a porous structure having a number of pores. And, there is provided a laminate of multiple wicks having different pore sizes. The patent describes that wicks having different pore sizes have difference capillary pressures, generating a temperature gradient between the inside and outside of the laminate of wicks, and thus, the flow of the working fluid can be smoothened. However, similarly to the above-mentioned JP-A Hei 10 (1998)-160368, this patent still has a problem of small pumping force. Therefore, the pumping force cannot adequately respond to load changes, and thus a stable cooling capacity cannot be obtained.